When an energy particle included in sunlight called a photon hits an i-layer, an electron and a positive hole (hole) are created due to a photovoltaic effect. As a result, the electron moves towards an n-layer, while the positive hole moves towards the p-layer. A solar cell is an element which extracts the electron, which was created by this photovoltaic effect, from an upper electrode and a back surface electrode, and converts light energy to electric energy.
FIG. 10 is a simplified cross sectional diagram of an amorphous silicon solar cell. A solar cell 100 is created by a layering of a glass substrate 101, which makes up the front surface; an upper electrode 103 comprising a zinc oxide series transparent conductive film provided on the glass substrate 101; a top cell 105 comprising an amorphous silicon; an intermediate electrode 107 comprising a transparent conductive film provided between a top cell 105 and a bottom cell 109 described later; a bottom cell 109 comprising a microcrystal silicon; a buffer layer 110 comprising a transparent conductive film; and a back surface electrode 111 comprising a metal film. (Refer to Patent Document 1, for example.)
The top cell 105 is a three-layered structure comprising a p-layer (105p), an i-layer (105i), and an n-layer (105n). Among these, the i-layer (105i) comprises amorphous silicon. In addition, similar to the top cell 105, the bottom cell 109 is also a three-layered structure comprising a p-layer (109p), an i-layer (109i), and an n-layer (109n). Among these, the i-layer (109i) comprises microcrystal silicon.
According to such a solar cell 100, the sunlight entering from the glass substrate 101 side is reflected at the back surface electrode 111 after passing through the upper electrode 103, the top cell 105 (p-i-n layer), and the buffer layer 110. Certain configurations are made to the solar cell in order to enhance the effectiveness of making a conversion to light energy by reflecting the sunlight at the back surface electrode 111, or by providing a structure called a texture on the upper electrode 101 so as to achieve a prism effect, which elongates the light path of the incident sunlight, and by achieving an effect to confine light. The buffer layer 110 is provided to prevent the dispersion of the metal film used in the back surface electrode 111. (Refer to, Patent Documents 2 and 3, for example.)
The wavelength band used for the photovoltaic effect differs depending on the device structure of the solar cell. However, at any event, it is necessary that the transparent conductive film, comprised in the upper electrode, have a characteristic such that light is passed through in order to absorb the light at the i-layer. It is also necessary that the transparent conductive film have electrical conductivity so as to extract the electron created by the photovoltaic power. As a result, an FTO, which is obtained by adding fluorine to SnO2 as an impurity, as well as a ZnO series oxide semiconductor thin film are used. Similarly, it is necessary that the buffer layer have a characteristic passing through a light, which reflects at the back surface electrode in order to be absorbed by the i-layer, and a light, which was reflected by the back surface electrode. It is also necessary that the buffer layer have electrical conductivity so as to transport the positive hole to the back surface electrode.
Generally speaking, the three elements that a transparent conductive film used in a solar cell is required to have as a characteristic are electrical conductivity, optical properties, and a textured structure. First, concerning the first characteristic, electrical conductivity, a low electrical resistance is required to extract electricity which was generated. Generally speaking, the FTO, used as a transparent conductive film for solar cells, gains electrical conductivity by replacing O with F, by adding F to the SnO2 at the transparent conductive film created by the CVD. Further, a ZnO series material, which is widely regarded as a post-ITO, may create a film by sputtering. Electrical conductivity is obtained by adding to ZnO, material comprising Al and Ga as well as oxygen deficiency.
Second, since a transparent conductive film for solar cells is primarily used at an incident light side, an optical property is required such that a wavelength band, absorbed by the electricity generating layer, is passed through.
Third, a textured structure is necessary to dissipate light so that sunlight is effectively absorbed by the electricity generating layer. Normally, each of the thin films of the oxidized indium series (for example, In2O3) created by a sputtering process, oxidized zinc series (for example, ZnO), or oxidized tin series (for example, SnO2), have a surface condition which is too flat to disperse a wavelength region which will be absorbed by the solar cell. Therefore, it is necessary to conduct a texture forming procedure by wet etching and the like.